In recent years, techniques to form thin film transistors (TFT) using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface have attracted attention. Thin film transistors are widely applied to electronic devices such as ICs and electro-optic devices and are particularly expected to be rapidly developed as switching elements of image display devices. Various metal oxides are used for a variety of applications. An indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include a tungsten oxide, a tin oxide, an indium oxide, and a zinc oxide. A thin film transistor in which a channel formation region is formed using such metal oxides having semiconductor characteristics is known (Patent Documents 1 and 2).
Furthermore, TFTs using oxide semiconductors have high field effect mobility. Therefore, driver circuits of display devices and the like can also be formed using the TFTs.